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The interpretation of some of our taxa as representatives of particular eucrustacean taxa requires a "short" intro. Sorry for that.

1. Eucrustacea

1.1. Phylogeny

The crown group of Crustacea, Eucrustacea is considered as the sister group of the †Phosphatocopina, and there must be characters to confirm this, as given below, not just a statement. And these characters must be demonstrated to fit. This is another problem because many authors like to state things but do not discuss the possible character conflicts. Even more, many do not like to discuss characters proposed by others.

Again, a so-called crown group embraces all taxa with living descendants. This implies that this taxon is monophyletic per se, originating from a single stem species, but it also omplies that it contains fossil taxa. Such knowledge seems to have gone lost, so that some authors even try to "proof" the monophyletic status of a crown-group and others think that the descendants alone constitute the crown and that there is a stem lineage of unknown taxa from the last node to the last common stem species, a pure construct to cloud things.

Since "crown" is a convention, and as there is always a next phylogenetically earlier branch with living descendants, it is simply at odds to discuss fossils as stem-groups of something or even to distinguish them terminologically. Sister taxa are simply equal to extant taxa, no matter how many species involved, and there are, as Ax (1995) stated, only two taxa in the Natural System, the species and the supra-specific taxon. He himself, subsequently departs from his clear way and neglects fossils wherever he can, but this does not change anything from the fact that sister taxa are or were species, no matter of whether they are currently living or only known from fossils. Therefore, again: All branches have to be treated equally, and it is only a matter of resolution if we, first-hand, are not able to clarify a bunch of taxa so leaving them as unbranched "stem lineage" derivatives until we know it better. This is true for example for our crustacean stem taxa, but perhaps we can do better in the future and resolve at least part of this early part of the crustacean system.

Eucrustacea can be distinguished from †Phosphatocopina by two autapomorphies:

  • the orthonauplius and
  • the maxillula = first maxilla = maxilla 1, a specialisation of the first post-mandibular limb.

Both features are lacking in the Phosphatocopina, better to say, Phosphatocopina retain the original euarthropod head larva and the "maxillula" is not distinguished from the succeeding trunk limbs, so these features being plesiomorphic. Likewise do Eucrustacea lack all those features developed in the evolutionary lineage of the Phosphatocopina.

For synapomorphies of Eucrustacea +
Phosphatocopina see the
autapomorphies of Labrophora

Eucrustacea consist, in our view, of the two sister taxa Entomostraca and Malacostraca. Both taxa can be founded on a set of autapomorphic characters, but we have to note that our interpretations are incongruent with suggestions by most other workers – not least because our features are not even discussed by these. And if discussed as in Boxshall (2007), these are not based on the information we presented.

1.2. In-group taxa

1.2.1. Entomostraca

Cephalocarida and Maxillopoda share with the Branchiopoda a number of features, named below as autapomorphies of Entomostraca. Also the new taxon Yicaris dianensis possesses these features strengthening the Entomostraca concept. Cephalocarida and Yicaris possess, plesiomorphiccally, 5- or 6-segmented endopods on the maxilla 2 and all trunk limbs (this number is invariably 5 in the Malacostraca, and has to be considered as a ground-pattern feature of this taxon [except a few in-group modifications]. In Branchiopoda (including Rehbachiella kinnekullensis) and in the Maxillopoda this number is smaller – 4 or less. Cephalocarida and Yicaris are, at present, regarded as the sister taxa of an unnamed taxon N.N., which includes the Maxillopoda and Branchiopoda (share an endopod on all postmaxillulary limbs being composed of four or fewer articles and an oval structure on the head shield named dorsal or neck organ). The position of Yicaris remains uncertain at present because we have not found a clear synapomorphy with either Maxillopoda or Branchiopoda. Symplesiomorphies don't count here...........

  • Autapomorphies of Entomostraca:

    • presence of four endites on the maxillula (= first maxilla; brush-limb, so-called mouthpart) = proximal endite + 3 similarly setiferous endites on basipod
      • plesiomorphic state: proximal endite + one endite on median side of basipod
    • presence of a set of true ring-shaped abdominal segments lacking limbs and according musculature
      • ples.: no such apodous segments
    • lack of a mandibular palp (basipod and rami) in the adult
      • ples: presence of a palp comprising basipod, endopod and exopod (as in phosphatocopines)
        not to be mismatched with the most likely secondary situation in maxillopods and the larval spituation in fossils like, e.g. Rehbachiella or Yicaris
    • presence of a special, elongated, fleshy basipod on all post-maxillulary limbs with proximal endite + up to 7 lobate setiferous endites medially
      • ples: basipod lacking such medidan basipodal endites, only having the proximal endite + 1 basipodal endite as in phosphatocopines

There are two more features, but which have to be investigated in detail still; both arer related to the basipod design:

    • basipods C-shaped curved backwards forming an open feeding- and locomotory apparatus.
    • basipods with 3 subdivisions.

1.2.1.2. Cephalocarida, to be added

1.2.1.3. Yicaris, see here

1.2.1.4. Maxillopoda

Walossek & Müller (1998) present as autapomorphic features in the ground pattern of the maxillopodan stem species:

  • the tagmosis comprising a head, a seven-segmented thorax (last as a genital segment – not abdominal! – in extant taxa), and an abdomen made of four segments plus the telson with furcal rami (telson and furca are plesiomorphic),
  • reduction of limb setation and elongation of the thoracopods in parallel with a more swimming mode of life,
  • and possibly the paedomorphic retention of a larval mandible with basipod and rami because ontogeny is terminated at a stage with 7+4 trunk segments.

To me is seems very likely that Maxillopoda evolved by paedomorphosis. It may be possible that the stem species was an active swimmer already with a slightly reduced feeding setation on the trunk legs. Within the Maxillopoda, trunk limbs lost their feeding setation at the basipods completely and turned into special swimming limbs with setation only on their endo- and exopods. This holds also for the filtratory limbs of the sessile barnacles.

The taxon Maxillopoda comprises the copepodan lineage with Copepoda, Mystacocarida and Skaracarida. The ground pattern of the stem species of the copepodan lineage, Copepodoida, includes autapomorphies, such as:

  • a special cephalo-maxillipedal feeding system with the 1st thoracopods being modified to maxillipeds
  • a specific subdivision of limb basis (traditionally the protopod) of maxillulae, maxillae, and maxillipeds
  • the loss of compound eyes,
  • possibly the reduction of the head shield margins is another shared character of the copepodan line.

and the thecostracan line with Bredocaris, the Tantulocarida, the Ascothoracida, the Facetotecta (Y-larvae) and the Cirripedia with the Acrothoracica and the Thoracica. The ground pattern of the stem species of this clade, the Thecostraca s. l. included autapomorphies, such as:

  • a very specific larval development = condensed series, with six pseudo-naupliar stages which develop the whole 7-segmented trunk (not 6, as in traditional views) plus the maxillary segment without external segmentation and development of functional limbs.

Plesiomorphic features of the thecostracan lineage: for example the well-developed head shield, the presence of compound eyes, and no specialized post-maxillary mouthparts. Plesiomorphic features of the copepodan lineage: for example the strictly anamorphic ontogeny.

Uncertain candidate taxa are (still) the Ostracoda and Branchiura but which both have maxillopod features (eye structures) that may serve to validate their association with this taxon. Boxshall in Liège 1997 and in his contribution in the arthropod relationship book (London conference volume) pointed to the possible relationships of these with the taxa of the copepodan line, as characterized above. This would imply that, as is to be expected, the stem line of the copepod clade still had a nice shield and compound eyes as plesiomorphic features dating from the eucrustacean ground pattern.

1.2.1.5. Branchiopoda, to be added

1.2.2. Malacostraca

  • Autapomorphies of the Malacostraca are numerous. Here some significant examples:

    • first antenna (antennula) biflagellate (no real segments)
    • endopod of second antenna ending in a long flagellum (no real segments)
    • mandible with three-segmented palp, made of a basipod and two-segmented endopod (palp alone is plesiomorphic), exopod missing (in maxillopods the palp is present but a larval features, and, consequently, the multi-annulated exopod is present)
    • maxillula and maxilla blade-like, stem consisting of coxa and basipod, no lobate endites, only a row of spines or setae, endopod 3-segmented at most, exopod leaf-shaped (ples.)
    • 14 thoracic segments with limbs (ples.), divided into one set with 8 and a second with 6 segments
    • gonopods unvariably on thorax segments 6 (females) and 8 (males).

Malacostraca include the Leptostraca (Phyllocarida) and the Eumalacostraca (a2 exopod as a flap = scaphocerit).

1.3. Which fossils are eucrustaceans, which are entomostracans or malaconstracans?

Accordingly, we have to find synapomorphies with the one or other group to recognize systematic affinities of 'Orsten' fossils with these sister taxa or in-groups of them. Yet, there are no signs of malacostracan affinities of any of our 'Orsten' taxa. This taxon has a fossil record from the Oordovician onwards, but any records from the Cambrian is unjustified so far, based on misinterpretations/misunderstandings of according fossils.

The worst case is the Lower to Middle Cambrian Canadaspis perfecta. Several papers have been published now on this form refuting its relationship to Crustacea, possibly being not even a euarthropod, but still people (one in particular) do not stop to cite this form as evidence of phyllocarid malacostracans in the Cambrian.

Similarly bad is the story with the phosphatocopines mistaken/miscited as ostracodes – still upheld by some authors – and, therefore, a evidence of a fossil record of this taxon also down in the Cambrian. This DOES NOT exclude that we'll find them once because of the fact that if a member of a specific taxon has been found, this simply demands for the sister taxon to be present too!

Entomostracan affinities can be made clear for several of our taxa: Bredocaris admirabilis, Dala peilertae, Rehbachiella kinnekullensis, Skara anulata, minuta and huanensis, and for Walossekia quinquespinosa. These will be described in more detail below.

The newly described Yicaris dianensis Zhang, Siveter, Waloszek & Maas, 2007 (Nature 449, 595–598, or ask us for a PDF) is regarded as a member of the Eucrustacea because it shows exactly those characters discussed above. It is even a member of the Entomostraca, hence an in-group-member of Eucrustacea, because its postmandibular limbs have a very long basibod compared to the endopod, and the basipod shows a number of endites assumed to be part of the autapomorphies in the entomostracan ground pattern. Furthermore, the material at hand shows the significant reduction of the mandibular palp, although it is not fully reduced since we do not have the later stages, in which this occurs autapomorphically in entomostracan ontogeny.
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